Research highlights: Radar is a simple concept—radio waves are sent out and their time and power of return are calculated to determine the range, angle, velocity, and characteristics of objects off of which the radar beam bounces. Synthetic aperture radar, or SAR, bounces a microwave radar signal off Earth’s surface to detect physical properties. While the word “aperture” when used in reference to an optical instrument like a film camera refers to the size of an opening in a lens that lets light in, the term “aperture” in radar use refers to the antenna generating the microwave pulses. In general, the larger the radar antenna, the more information and better surface resolution the radar can produce. Since antenna size is limited on satellite instruments, scientists use the spacecraft’s motion along with advanced signal-processing techniques to simulate a larger antenna and create high resolution images. This is where the “synthetic aperture” comes from. Significant advantages of SAR are that it can create high resolution images without the need for illumination (such as from the sun) and can penetrate clouds, fog, tree canopies, or other obstructions to create these images. This makes SAR ideal for use in Earth observing satellites.

Dr. Xiaofeng Li uses SAR data to study a wide range of processes occurring in the atmosphere and ocean, including air-sea interactions, ocean surface winds, waves, coastal upwelling, oil seeps, and tropical cyclones. In fact, SAR has been used to observe tropical cyclones since the launch of the first satellite-borne SAR aboard NASA’s Seasat mission in 1978. SAR reveals visible tropical cyclone features like eye structure, rain bands, and arc clouds, as well as features that may not be visible, such as the presence of high winds within a cyclone’s eye. Dr. Li and his colleagues use SAR to better understand tropical cyclone morphology as well as to help determine physical parameters including wind speed and direction, rain rate, and eye location, all of which help improve cyclone tracking and intensity predictions.

In a recent study, Dr. Li and his colleagues combined SAR measurements of tropical cyclones with cloud pattern data from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument aboard the Terra and Aqua Earth observing satellites. The combination of these data allowed the team to study cyclone tilt, which is one indicator of storm intensity (strong, developing storms tend to tilt from the storm base to the storm top, generally tilting toward cooler upper-level air). For physical wind retrievals, conventional radar signals tend to produce ambiguous returns under hurricane conditions and very high winds. To alleviate this problem, Dr. Li and his colleagues developed high-wind retrieval algorithms using cross-polarization SAR measurements along with SAR-derived hurricane morphological information. These cross-polarization measurements exhibit less scattering at high wind speeds and allow for more accurate wind retrievals. The research team compared their SAR-derived hurricane wind measurements with wind measurements from the airborne Stepped-Frequency Microwave Radiometer (SFMR), which is part of NOAA’s Hurricane Research Division.

Dr. Li also uses SAR to develop algorithms for tracking changes in coastlines and the movement of oil seeps and spills. Since SAR images can be captured day or night and in almost all kinds of weather, long-term coastline changes can be observed and tracked over time easily. In addition, oil on the sea surface tends to damp, or flatten, surface capillary waves and make the sea surface smooth. These patches of oil-dampened water appear as dark features in SAR images and can be used to study ocean currents underneath the oil as these currents move the oil. Dr. Li and his colleagues used multiple SAR observations and oil drifting models to identify the movement of oil at sea and assess spots where oil might come ashore.

In another recent study, Dr. Li and his colleagues used SAR data to observe ocean waves called internal solitary waves (ISW), which are large amplitude ocean waves that tend to occur in areas where water density increases rapidly with depth. One area where these waves are common is the South China Sea. The specific waves Dr. Li and his colleagues studied are generated by internal tides occurring at the Luzon Strait in the South China Sea. These waves move westward toward mainland China at roughly 3 m/s toward the continental shelf. Dr. Li and his colleagues chose to look at how the Dongsha Atoll affected the movement of these internal waves. The atoll (a ring-shaped island made of coral) is located at the edge of the continental shelf roughly 450 km (about 280 miles) west of the Luzon Strait. As these undersea waves hit and move around the atoll, the incoming wave is split and then re-joins on the western side of the atoll. This change in wave direction is called “refraction.” The research team used SAR imagery to map ISW signatures around the atoll in order to understand their generation mechanisms, type, spatial distribution, propagation speed, refraction, and other processes.

Research findings: In his research into hurricane morphology, Dr. Li and his colleagues found that storm eye shapes in tropical cyclones can be categorized using SAR imagery and that stronger storms tend to have more symmetrical eyes. Also, heavy rain and atmospheric properties can interfere with the radar beam and cause false returns to appear over land and sea. Through the use of the high-wind retrieval algorithms developed by Dr. Li and his colleagues, the team was able to mitigate this beam interruption and reconstruct a complete tropical cyclone wind map, including both speed and direction.

In his work on oil tracking, Dr. Li and his colleagues developed new oil detection methods based on statistical and physical approaches. Using SAR images of oil slicks and seeps as tracers, the research team found that the movement of oil slicks and seeps in the Gulf of Mexico is directed by currents that are affected by the Earth’s rotation (Coriolis force). A sudden wind blowing over the water tends to cause surface water initially to move in the direction of the wind, but then turn this water to the right-hand side of the wind direction in the Northern Hemisphere because of the Coriolis force. Once this motion is established, the surface water continues moving in a circle. Oil is carried along with this water, and can be more easily tracked. Algorithms developed by Dr. Li and his colleagues as a result of this research have been used in NOAA’s daily oil slick monitoring operations.

Finally, in his research into internal waves near Dongsha Atoll, Dr. Li found unusual wave refraction patterns revealed by the SAR imagery. The research team noticed that after the wave hit the atoll and split, the wave closest to the atoll moved around the atoll in a circular pattern while the other side of the split wave continued moving west toward the continental shelf. Dr. Li and his colleagues found that this phenomenon was caused by the asymmetry of the tidal current near the atoll and hypothesize that the observed wave refraction is caused by changes in water depth around the atoll.